Just curious to what length of music game creators prefer?
I don't really know the answer to that, sorry.
Only thing I know is that I've seen some people express interest in ambient in their games (they didn't mention buying music, though), and that lately, in terms of games, I've been seeing a lot of 16-bit/PSX-style JRPGs and modern first-person horror games on Itch.
I prefer 2 - 3 minutes per track with some variation (key or instrument changes). I often end up mixing shorter 1 minute tracks to get a longer track with variation.
My personal preference is music with a strong melody line that does not sound like cheap electronic music. (I think that type of music tends to get more attention here - but not sure that translates into sales or downloads)